d
Negro jaws at the College of Surgeons.[2] He maintains that the
diminution of the jaw in civilized races can _only_ have been brought
about by inheritance of the effects of lessened use. But if English jaws
are lighter and thinner than those of Australians and Negroes, so too is
the rest of the skull. As the diminution in the weight and thickness of
the walls of the cranium cannot well be ascribed to disuse, it must be
attributed to some other cause; and this cause may have affected the jaw
also. Cessation of the process by which natural selection[3] favoured
strong thick bones during ages of brutal violence might bring about a
change in this direction. Lightness of structure, facilitating agility
and being economical of material, would also be favoured by natural
selection so far as strength was not too seriously diminished.

Sexual selection powerfully affects the human face, and so must affect
the jaws--as is shown by the differences between male and female jaws,
and by the relative lightness and smallness of the latter, especially in
the higher races. Human preference, both sexual and social, would tend
to eliminate huge jaws and ferocious teeth when these were no longer
needed as weapons of war or organs of prehension, &c. We can hardly
assume that the lower half of the face is specially exempt from the
influence of natural and sexual selection; and the effects of these
undoubted factors of evolution must be fully considered before we are
entitled to call in the aid of a factor whose existence is questioned.

After allowing for lost teeth and the consequent alveolar absorption,
and for a reduction proportional to that shown in the rest of the skull,
the difference in average weight in fifty European and fourteen
Australian male jaws at the College of Surgeons turned out to be less
than a fifth of an ounce, or about 5 per cent. This slight reduction may
be much more than accounted for by such causes as disuse in the
individual, human preference setting back the teeth, and partial
transference of the much more marked diminution seen in female jaws.
There is apparently no room for accumulated _inherited_ effects of
ancestral disuse. The number of jaws is small, indeed; but weighing them
is at least more decisive than Mr. Spencer's mere inspection.

The differences between Anglo-Saxon male jaws and Australian and
Tasmanian jaws are most easily explained as effects of human preference
and natural selection. We can h